It is possible to find out using mathematical methods whether a given integer is a prime number or not.
No, 21 402 is not a prime number.
For example, 21 402 can be divided by 2: 21 402 / 2 = 10 701.
For 21 402 to be a prime number, it would have been required that 21 402 has only two divisors, i.e., itself and 1.
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21 402 is an even number, because it is evenly divisible by 2: 21 402 / 2 = 10 701.
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A number is a perfect square (or a square number) if its square root is an integer; that is to say, it is the product of an integer with itself. Here, the square root of 21 402 is about 146.294.
Thus, the square root of 21 402 is not an integer, and therefore 21 402 is not a square number.
The square of a number (here 21 402) is the result of the product of this number (21 402) by itself (i.e., 21 402 × 21 402); the square of 21 402 is sometimes called "raising 21 402 to the power 2", or "21 402 squared".
The square of 21 402 is 458 045 604 because 21 402 × 21 402 = 21 4022 = 458 045 604.
As a consequence, 21 402 is the square root of 458 045 604.
The multiples of 21 402 are all integers evenly divisible by 21 402, that is all numbers such that the remainder of the division by 21 402 is zero. There are infinitely many multiples of 21 402. The smallest multiples of 21 402 are: